(RNN) – Some people say there is a lot of chaos on Capitol Hill, but the real Chaos is at the Department of Defense.

‘Chaos’ is the call sign of outwardly calm Defense Secretary James Mattis. And no, don't get it confused with "Mad Dog," which is his nickname. Mattis says he doesn’t know how he got that particular moniker hung on him.

A call sign is a name that is used in communications, for instance between the Secret Service and other agencies assigned to protect presidents and other high-ranking officials. For example, First Lady Jackie Kennedy was appropriately called "Lace," President Ronald Reagan was "Rawhide," Barack Obama was "Renegade," and Mitt Romney was "Javelin."

Mattis knows exactly how he came to be called ‘Chaos,’ and explained during the Air Force Association Air, Space and Cyber Conference in National Harbor, MD, this fall. The story drew plenty of laughter from the audience.

He said while he was a Marine Corps colonel in the Mojave Desert in the U.S. Southwest, he would often have what he thought were good ideas for the troops under his command.

He said one day he was walking by the office of his operations officer and noticed the word ‘chaos,’ written on his whiteboard.

After being pressed by Mattis for an answer of what the word meant, the officer said it stood for the "Colonel Has An Outstanding Solution."

“I liked what my irreverent troops had used there, so I adopted it as my call sign,” Mattis said.